1. Field of the Invention
This invention relates to wear-resistant tools for cutting and drilling and in particular to wear-resistant rotary cutting tools for machining metals and other materials.
2. Description of the Related Art
Almost every product made by the engineering industries requires at some stage in its manufacture the machining of metals or other materials. Typical machining operations include drilling, milling, reaming, thread cutting, slot cutting, and turning. Typically, these operations are carried out by automated machine tools which are fitted replaceably and often interchangeably with tool bits such as drill bits, end mills, thread taps, slot drills and reamers. The preferred shapes of the various cutting tools are well known in the art and will not be described further here.
The material from which the cutting tools are made must combine as far as possible the properties of resistance to deformation (hardness), resistance to fracture (toughness), and resistance to wear (durability). The most widely used materials for this purpose are steel and tungsten carbide. However, both steel and tungsten carbide are prone to wear in the operating environment and as a result cutting tools made of these materials need to be removed for sharpening or replacement quite frequently. The expense of sharpening or replacing the steel or tungsten carbide tool bits, and the machine tool downtime that is needed for their frequent replacement, add considerably to the total cost of the manufacturing process and interfere with the smooth operation of production lines.
Abrasive compacts are also well known in the art and are used extensively in industry for the abrading of various workpieces. They consist essentially of a mass of abrasive particles present in an amount of at least 70 percent, preferably 80 to 90 percent, by volume of the compact bonded into a hard conglomerate. Compacts are polycrystalline masses and can replace single large crystals in many applications. The abrasive particles of compacts are invariably ultra-hard abrasives such as diamond and cubic boron nitride. Compacts containing diamond abrasive particles are known in the art by the initials PCD. Compacts containing cubic boron nitride abrasive particles are known as PCBN.
Abrasive compacts generally contain a second phase or bonding matrix which contains a catalyst (also known as a solvent) useful in synthesising the particles. In the case of cubic boron nitride, examples of suitable catalysts are aluminium or an alloy of aluminium with nickel, cobalt, iron, manganese or chromium. In the case of diamond, examples or suitable catalysts are metals of Groups VIII of the Periodic Table such as cobalt, nickel or iron or an alloy containing such a metal.
As is known in the art, diamond and cubic boron nitride compacts are manufactured under conditions of temperature and pressure at which the abrasive particle is crystallographically stable.
Abrasive compacts may be bonded directly to a tool or shank for use. Alternatively, they may be bonded to a backing such as a cemented carbide backing prior to being mounted on a tool or shank. Such backed compacts are also known in the art as composite abrasive compacts.
The backing will typically be made of cemented carbide such as cemented tungsten carbide, cemented tantalum carbide, cemented titanium carbide or a mixture thereof.
Hitherto, machining tools comprising PCD or PCBN cutting edges have generally been made from flat pieces of PCD or PCBN or their composites. Tools with more complex shapes have generally been made by brazing flat pieces of PCD or PCBN/tungsten carbide composite onto tungsten carbide tool bodies followed by machining the body and composite together to form the desired tool. The limitations that result from using a planar geometry for the PCD layer can readily be appreciated by considering the case of the most widely used machining tool: the twist drill bit.
At present, standard twist drill bits having PCD or PCBN cutting surfaces are mainly of two types, as described in U.S. Pat. Nos. 4,679,971 and 4,527,643. In the first type, a solid disc of PCD or PCBN/tungsten carbide composite is brazed to the tip of a coaxial tungsten carbide shank of similar diameter, resulting in a cylindrical blank tipped at one end with a thin layer of PCD or PCBN. This blank is then machined to the desired drill bit shape. Since the tip of a standard drill bit is preferably pointed with a tip angle of approximately 118.degree., it follows that drill bits made in the above way are characterised by a PCD or PCBN layer at the cutting edge whose thickness decreases linearly with increasing distance from the rotary axis of the drill bit. Typically, the PCD or PCBN layer near to the distal edge of the tip is extremely thin and therefore relatively weak. Furthermore, resharpening the tool will remove this thin layer entirely.
Drill bits of diameter greater than 5 mm are generally furnished with PCD or PCBN cutting edges by forming a suitable blank of cemented tungsten carbide or similar material, cutting one or more slots into the tip of the blank, and brazing one or more flat pieces of PCD or PCBN/tungsten carbide composite into the slot or slots, followed by machining the blank and inserts jointly to form a drill bit with cutting edges composed of PCD or PCBN. This method of manufacture is expensive. Furthermore, drill bits made in this way are characterised by a non-ideal tip configuration imposed by the flat PCD or PCBN insert. The cutting edges are at the wrong angle and not properly aligned relative to each other, which results in uneven machining and excessive heat generation. The imperfect tip configuration interferes with the removal of swarf.
Drill bits with diameters greater than 5 mm made by the processes described above are currently in use. Their superior wear resistance outweighs their cost and disadvantageous properties. Typically, holes are drilled initially with a PCD or PCBN tipped drill, which drills unevenly for the reasons set out above, and the holes are then given a smooth finish with a conventional steel or tungsten carbide drill.
U.S. Pat. No. 4,713,286 discloses twist drills in the diameter range 0.15-3.2 mm for use in printed circuit board manufacture. These drills are machined from drill blanks, which blanks comprise one or more veins of PCD or PCBN bonded in situ to a cemented tungsten carbide body, the veins of PCD or PCBN positioned to extend longitudinally from the conically shaped point region of the drill blank. Once again the machining required to form the twist drill is complex, and the twist drills provided in this way have non-ideal tip configurations because the cutting edges on opposite sides of the drill tip are not aligned.
The absence of satisfactory PCD or PCBN tipped drills and other rotary cutting tools represents a serious and long-felt deficiency in machine tool technology in view of the outstanding importance of rotary machining operations in most manufacturing processes.
Accordingly, it is an object of the present invention to provide twist drill bits having cutting edges of PCD or PCBN with improved cutting properties, improved toughness and improved durability relative to existing cutting tools of this type.
It is a further object of the present invention to provide slot drills, end mills, thread taps and reamers having the advantageous properties listed above.
It is a further object of the present invention to provide methods of manufacturing the twist drill bits, slot drills, end mills, thread taps and reamers according to the present invention.